Method and device for measuring the temperature of a molten metal bath

ABSTRACT

A method is provided for measuring a temperature of a molten metal bath by an optical fiber surrounded by a cover. The optical fiber is immersed in the molten bath, and the radiation absorbed by the optical fiber in the molten bath is fed to a detector, wherein the optical fiber is heated when immersed in the molten bath. The heating curve of the optical fiber has at least one point P(t 0 , T 0 ), wherein the increase ΔT 1  in the temperature T of the optical fiber over the time Δt in a first time interval t 0 −Δt up to the temperature T 0  is smaller than the increase ΔT 2  in the temperature of the optical fiber over the time Δt in an immediately following second time interval t 0 +Δt.

BACKGROUND OF THE INVENTION

The invention relates to a method for measuring a parameter, inparticular a temperature, of a molten bath, in particular a molten metalbath, by an optical fiber surrounded by a cover, the optical fiber beingimmersed in the molten bath and the radiation absorbed by the opticalfiber in the molten bath being fed to a detector, wherein the opticalfiber is heated when immersed in the molten bath. Furthermore, theinvention relates to a device for measuring a parameter, in particular atemperature, of a molten bath, in particular a molten metal bath, withan optical fiber having a cover and a detector connected to the fiber,wherein the cover surrounds the fiber in a plurality of layers.Parameters in the sense of the invention may also be, for example, theheight of the bath or the composition, in other words the proportion ofcomponents. It is also possible to measure in other molten baths, suchas molten salt, cryolite or glass baths.

A method of this kind is known, for example, from Japanese patentapplication publication No. JP 11-118607. This describes how an opticalfiber is used for measuring temperature in molten metal baths. Theoptical fiber is unwound from a spool and fed to the molten metal baththrough a feed pipe. The radiation absorbed by the optical fiber isevaluated by a detector. Suitable optical fibers are known, for example,from Japanese patent application publication No JP 10-176954. The fiberdescribed there is surrounded with a spacing by a metal tube. Arrangedaround the tube is a tube made of an insulating material, which in turnis surrounded by an outer metal tube. This structure prevents the innermetal tube from melting too quickly. The tube made of an insulatingmaterial contains carbon particles, so the inner metal tube does notmelt until the corresponding tube portion is immersed in the moltenmetal bath. The fiber is immersed in the molten metal bath and trackedat a previously determined speed, so that it is possible to continuemeasuring, even if the tip of the fiber is destroyed. A similar opticalfiber for measuring temperature is disclosed in Japanese patentapplication publication No JP 7-151918. Here, the optical fiber issurrounded by a protective metal tube surrounded by a layer of plasticmaterial.

Furthermore, multi-layered wires are known, which are used in steelworksto introduce doping substances selectively into the molten steel bath(for example, in DE 199 16 235, DE 37 12 619, DE 196 23 194, U.S. Pat.No. 6,770,366).

BRIEF SUMMARY OF THE INVENTION

The object of the invention is to improve the measuring of parameters inmolten baths using optical fibers.

The optical fiber is by nature heated when it is immersed in the moltenbath or when it approaches the molten bath or the layer of slag abovethe molten bath (for example 1. on molten steel baths). The heatingrelates, in particular, to the tip or the immersion end of the opticalfiber. The optical fiber, the light-conducting element of which isusually quartz glass, has to be regularly replaced where the tip isconcerned, for example, in molten steel baths, as quartz glass cannotwithstand the high temperatures of the molten steel bath for long. Themethod according to the invention correspondingly relates to the frontpart of the optical fiber in each case, which is immersed in the moltenbath or a layer of slag above it. The heating curve of the optical fiber(which represents the increase in the temperature T as a function of thetime t) has, according to the invention, at least one point P(t₀, T₀),wherein the increase ΔT₁ in the temperature of the optical fiber overthe time Δt in a first time interval t₀−Δt up to temperature T₀ issmaller than the rise ΔT₂ of the temperature of the optical fiber overthe time Δt in an immediately following second time interval t₀+Δt.

A temperature course of this kind means that from a particular point intime onwards the heating curve in principle has a bend (quasidiscontinuity), at which the heating speed considerably increasescompared with the previous course. It has been proved that a mechanicalmovement of the optical fiber or its immediate environment takes place,the magnitude of which depends on the size of the change in the heatingspeed and the shortness of the corresponding time interval. The greaterthe change in the heating speed and the smaller the time interval Δt is,the greater is the mechanical movement of the fiber or its immediateenvironment at the time of this quasi erratic change in the heatingcurve. This movement assists the immersion of the optical fiber in themolten bath and the replacing of the tip of the optical fiber, which ispractically pushed off by the suddenly arising movement (vibration), soa new end of the glass fiber, not yet damaged by high temperatures, canbe tracked.

The increase ΔT₂ in the temperature T after the second time intervalt₀+Δt is at least 5 times, preferably at least 10 times, more preferablyat least 20 times as large as the increase ΔT₁ in the temperature in thefirst time interval t₀−Δt. In particular, a 50 times or even better 100times greater increase in the temperature in the second time interval ispreferred. The duration Δt of the two time intervals shouldadvantageously be at the most 500 ms, preferably at the most 200 mslong.

It is appropriate that the temperature T₀ of the optical fiber allocatedto the point in time t₀ between the two time intervals is a maximum of600° C., preferably a maximum of 200° C., more preferably a maximum of100° C. The temperature of the actual optical fiber in the narrowersense, in other words the temperature of the quartz glass should beconsidered in this. The lower this temperature T₀, on which the changein the heating speed is based, the stronger and more effective thischange can be.

The speed at which the optical fiber is immersed in the molten metalbath or fed thereto corresponds to the speed at which the vitreousstructure of its tip is destroyed, so new glass fiber material isconstantly fed which is suitable for receiving and passing on radiation,without radiation losses arising owing to a destroyed fiber structure.

According to the invention, the device for measuring a parameter, inparticular a temperature, of a molten bath, in particular a molten metalbath, with an optical fiber, having a cover, and a detector connected tothe fiber, wherein the cover surrounds the fiber in a plurality oflayers, is characterised in that one layer is designed as a metal tubeand an intermediate layer arranged beneath it is formed from a powder ora fibrous or granular material, wherein the material of the intermediatelayer surrounds the fiber in a plurality of separate parts. The featureaccording to which the material of the intermediate layer surrounds thefiber in a plurality of separate parts means in the sense of theinvention that the construction in multiple parts exists in theoperating state, in other words during or after immersion in the moltenbath to be measured. In this case temperatures of at least 1000° C.,preferably at least 1400° C., are encountered. In this state, a bindingagent possibly used during manufacture between the parts of theintermediate layer is dissolved or burned, so the individual parts donot or largely no longer adhere to one another. The parts may formeither small particles or else larger cohesive units, such asconglomerates or, for example, shells arranged round the fiber. Thematerial of the intermediate layer is therefore not rigid overall but atleast to a limited extent movable in itself.

An intermediate layer of this kind is heated during the immersion of theoptical fiber in the molten metal bath or a layer of slag above it. Ithas surprisingly been proved that the combination of a metal tube and anintermediate layer arranged beneath it, made of a powder or a fibrous orgranular material, results in this material of the intermediate layersuddenly expanding greatly during heating, from a particular pointonwards, in a heating curve with the presence of gases, namely if themetal tube is heated so much that it can no longer withstand thepressure which arises inside the metal tube, because of the expansion ofthe gases of the intermediate layer conditional on heat. In this case,fast rising stresses form inside the metal tube, until it suddenlycracks or is destroyed in some other way, so that the cover of theoptical fiber moves away from the fiber practically explosively. Ingeneral, the device according to the invention is characterised in thatduring or after the destruction of the metal tube, the intermediatelayer very quickly disintegrates as a layer, its parts moving away fromthe fiber. In this way, on the one hand the optical fiber is veryquickly and suddenly exposed to the molten metal bath at its immersedend and on the other hand the advance of the tip of the optical fiberinto the molten metal bath is made considerably easier.

The intermediate layer is preferably formed of silicon dioxide,aluminium oxide or a material refractory to a molten steel bath or aninert material. The material of the intermediate layer is not rigid initself, but the individual material particles are movable in respect ofone another, so that on the one hand the cover with the optical fiber isas flexible as possible, and on the other hand the erratic nature of thebursting or releasing of this material is guaranteed. The cover may havean outer layer of metal, in particular of zinc, of ceramic paper,cardboard or plastic material.

The cover preferably has a vibrator or a vibrator is arranged on or nextto the cover, to improve the release of the material of the cover fromthe optical fiber or the removal (breaking off) of the destroyed tip ofthe optical fiber. The vibrator may also be formed by the material ofthe intermediate layer, as it has been proved that the particles of thematerial of the intermediate layer move towards one another on heating,this movement taking place partially erratically, so vibrations arisewithin this material or within the intermediate layer.

The vibrator may be formed of a material forming gas between 100° C. and1700° C. (e.g. plastic material or other material, which burns or givesoff gas in this temperature range). It can also be appropriate thatarranged between the vibrator and the cover there is an intermediatespace which is smaller than the oscillation amplitude of the vibrator.In particular, if the vibrator is arranged outside the cover, it actsmechanically on the cover periodically, so that the vibration isoptimally transmitted by these beats. A further advantageous option isthat the outside of the cover has irregularities arranged in successionin the longitudinal direction, into which an obstacle arranged next tothe cover acts, particularly on a fiber guiding device, so that when theoptical fiber is advanced a vibration is generated.

Additionally, the optical fiber may be surrounded by a metal sleeve asan inner layer. The layers of the cover may be arranged directly againstone another in each case, the layer on the inside preferably restingdirectly against the optical fiber. The metal tube of the cover and alsothe metal sleeve are advantageously made of steel, particularly if thedevice is used for measuring in a molten steel bath or a molten ironbath. In general, the melting point of the material of the metal tube orthe metal sleeve should correspond to the melting point of the moltenmetal bath to be measured.

The device according to the invention has, generally speaking, a coverfor the quartz glass fiber, which on immersion into the molten metalbath is discontinuously destroyed. In this way, the optical fiber iskept for a relatively long time at a very low temperature. From aparticular temperature onwards the fiber is heated erratically to theequilibrium temperature in the molten metal bath, so that themeasurement can then take place very quickly before the optical fiber orits end immersed in the molten metal bath is destroyed. By continuouslytracking the fiber into the molten bath at the same speed at which itsimmersed end is destroyed, there is always usable fiber materialavailable in the molten metal bath for measuring. The tip of the fiberis continuously destroyed, so that the erosion face of the fiber ispractically stationary. For this purpose, the fiber or its immersion endshould reach the bath temperature at the moment its degradation begins(this so-called critical speed is in this case therefore identical tothe erosion speed at which the erosion face of the fiber moves). If theerosion speed is lower than the critical speed, the fiber is destroyedbefore it reaches the bath temperature.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown. In thedrawings:

FIG. 1 is a graph shows a heating curve of an optical fiber plottingtemperature against time for a method according to one embodiment of theinvention, and illustrating the point where the change in heating speedoccurs, as discussed above in the Brief Summary of the Inventionsection;

FIG. 2 is a schematic illustration of a device according to oneembodiment of the invention;

FIG. 3 is a schematic illustration of a mechanical vibration option forthe device;

FIGS. 4 a to 4 c are schematic illustrations of the device according todifferent embodiments of the invention, each with detector;

FIGS. 5 a to 5 d are schematic cross-sections of various embodiments ofa fiber with a cover for the device; and

FIG. 6 is a detailed illustration of the fiber in cross-section.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 the temperature/time course is illustrated for the immersionof an optical fiber in a molten steel bath according to the method ofthe invention. The immersion speed of the quartz glass fiber with coverin the molten steel bath is equal to its destruction speed (erosionspeed), so that the erosion face is quasi stationary in the molten metalbath. This speed corresponds to the critical speed, so that the opticalfiber on its destruction face has reached the bath temperature.

Inside its covering the quartz glass fiber itself has only a very smallincrease in temperature over a long period of time. At a particularpoint in time its cover is suddenly removed, so that its temperatureincreases in a short time very steeply until it reaches the equilibriumtemperature in the molten steel bath.

In FIG. 2 a melting tub 1 with a molten steel bath 2 is illustrated. Anoptical fiber arrangement 3 is immersed in this bath. The optical fiberarrangement 3 has, above the molten metal bath, an outer covering 4,which serves for easier propulsion by a propulsion device 5. At the endof the covering 4 facing the molten steel bath 2 a vibrator 6 isarranged, which beats on the covering 4 at short intervals, so that thecover of the quartz glass fiber is suddenly destroyed by the vibrationgenerated, as soon as it has reached a predetermined temperature. Atthis point the temperature of the outer steel cover is already veryhigh, the powder arranged between the quartz glass fiber and the outersteel cover or the gas contained in the intermediate layer has greatlyexpanded and, assisted by the mechanical effect of the vibrator 6,explodes the steel cover which is under thermo-mechanical stresses inany case. As a result, the quartz glass fiber is immediately exposed tothe temperature of the molten steel bath, so that it heats up extremelyquickly to the equilibrium temperature. The intermediate layer is formedof silicon dioxide powder or aluminium oxide powder.

FIG. 3 shows an optical fiber arrangement 3 with a cover, which has onits outside irregularities arranged in succession in the longitudinaldirection. The optical fiber arrangement 3 is guided by a guide sleeve7, which has inside it a support element 8, along which the opticalfiber arrangement 3 is guided. On the side of the optical fiberarrangement 3 opposite the support element 8 an edge of the guide sleeve7 is tangent-bent inwardly, so that at this point it forms an obstacle9. This obstacle 9 engages in the irregularities of the optical fibercover, so that the optical fiber arrangement 3 is constantly vibratedduring its advance movement.

FIG. 4 a shows an optical fiber arrangement 3, in which the fiber 10, aquartz glass fiber, is surrounded by a steel tube 11. Inside the steeltube 11 is arranged an intermediate layer 12 made of aluminium oxidepowder. The quartz glass fiber 10 is connected to a detector 13 at itsend facing away from the immersion end of the optical fiber.

In FIG. 4 b a similar arrangement is illustrated, and here the quartzglass fiber 10 is surrounded by a metal sleeve 14. Cooling gas can beconducted through the metal sleeve 14, which is guided out of the steeltube 11 at the detector-side end of the optical fiber arrangement 3, sothat the quartz glass fiber 10 is additionally cooled.

FIG. 4 c shows an arrangement, likewise similar to FIG. 4 a, of anoptical fiber 3. The intermediate space between the steel tube 11 andthe quartz glass fiber 10 is divided into a plurality of chambers withthe aid of cardboard discs 15 arranged perpendicular to the opticalfiber 10. The cardboard discs 15 serve on the one hand to stabilize theintermediate layer 12. They stabilize, in particular, the powder of theintermediate layer 12 during the destruction of the optical fiberarrangement 3, running in the longitudinal direction. On the other hand,during the burning of the cardboard discs 15, taking place because ofthe heating, an additional discontinuity/disruption is generated, whichcontributes to exposing the quartz glass fiber 10 quickly to the moltenmetal bath, so that it heats up very quickly after the destruction ofthe cover.

In FIGS. 5 a to 5 d several options are illustrated for stabilizing thequartz glass fiber 10 in the center of the cover of the optical fiberarrangement 3. According to FIG. 5 a, the steel tube 11 is bent in sucha way that it forms in one piece a concentrically arranged inner tube16, which is connected to the outer steel tube 11 by a web 17 runningalong the cover. The outer steel tube 11 is welded together at a seampoint 18 and has a wall thickness of approximately 0.5 mm. The quartzglass fiber 10 is arranged in the inner tube 16.

In the embodiment according to FIG. 5 b the quartz glass fiber 10 isarranged centrally in the material of the intermediate layer 12.

FIG. 5 c shows a further embodiment of the optical fiber arrangement 3,similar to FIG. 5 a. Here, though, the steel tube 11 is composed of twohalves, in each case jointly forming two webs 17, by which the quartzglass fiber 10 is centrally locked.

The embodiment according to FIG. 5 d is similarly constructed. Itadditionally has a second outer steel tube 19, which holds together thesteel tube 11 formed from two shells. The wall of the two steel tubes11, 19 can be reduced correspondingly in respect of the otherembodiments and amounts in each case to approximately 0.25 mm. A singlewelding at the seam point 20 is required.

FIG. 6 shows a fiber cross-section in detail. The quartz glass fiber 10is surrounded at a minimal distance by a steel casing 21, so thatdifferent expansions of the two materials on heating are possible, andthe quartz glass fiber 10 is nevertheless stabilized. Between the steelcasing 21 and the steel tube 11 is arranged an intermediate layer 12made of aluminium oxide particles. The steel tube 11 is rolled from ametal sheet and closed by a fold 23.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A method for measuring a parameter of a molten bath by an opticalfiber surrounded by a cover, the method comprising immersing the opticalfiber in the molten bath, such that the optical fiber is heated whenimmersed in the molten bath, feeding the radiation absorbed by theoptical fiber in the molten bath to a detector, and plotting the heatingcurve of the optical fiber to determine at least one point P(t₀, T₀)where the increase ΔT₁ in a temperature T of the optical fiber over timeΔt in a first time interval t₀−Δt up to the temperature T₀ is smallerthan an increase ΔT₂ in the temperature of the optical fiber over thetime Δt in an immediately following second time interval t₀+Δt.
 2. Themethod according to claim 1, wherein the parameter is a temperature ofthe molten bath.
 3. The method according to claim 1, wherein the moltenbath comprises a molten metal bath.
 4. The method according to claim 1,wherein the increase ΔT₂ in the temperature in the second time intervalt₀+Δt is at least 5 times as large as the increase ΔT₁ in thetemperature in the first time interval t₀−Δt.
 5. The method according toclaim 4, wherein the increase ΔT₂ in the temperature in the second timeinterval t₀+Δt is at least 10 times as large as the increase ΔT₁ in thetemperature in the first time interval t₀−Δt.
 6. The method according toclaim 5, wherein the increase ΔT₂ in the temperature in the second timeinterval t₀+Δt is at least 20 times as large as the increase ΔT₁ in thetemperature in the first time interval t₀−Δt.
 7. The method according toclaim 6, wherein the increase ΔT₂ in the temperature in the second timeinterval t₀+Δt is at least 50 times as large as the increase ΔT₁ in thetemperature in the first time interval t₀−Δt.
 8. The method according toclaim 7, wherein the increase ΔT₂ in the temperature in the second timeinterval t₀+Δt is at least 100 times as large as the increase ΔT₁ in thetemperature in the first time interval t₀−Δt.
 9. The method according toclaim 1, wherein the time Δt is at most 500 ms long.
 10. The methodaccording to claim 9, wherein the time Δt is at most 200 ms long. 11.The method according to claim 1, wherein the temperature T₀ of theoptical fiber allocated to the point of time t₀ between the two timeintervals is a maximum of 600° C.
 12. The method according to claim 11,wherein the temperature T₀ of the optical fiber allocated to the pointof time t₀ between the two time intervals is a maximum of 200° C.
 13. Adevice for measuring a parameter of a molten bath, the device comprisingan optical fiber, a cover laterally surrounding the fiber, and adetector connected to the fiber, wherein the cover surrounds the fiberin a plurality of layers, one layer comprising a metal tube and anintermediate layer arranged beneath the metal tube, the intermediatelayer comprising a powder or a fibrous or granular material, wherein thematerial of the intermediate layer surrounds the fiber in a plurality ofpieces.
 14. The device according to claim 13, wherein the parameter is atemperature of the molten bath.
 15. The device according to claim 13,wherein the molten bath comprises a molten metal bath.
 16. The deviceaccording to claim 13, wherein the intermediate layer comprises an inertmaterial, silicon dioxide, aluminium oxide, or a material refractory tothe molten bath.
 17. The device according to claim 13, furthercomprising an outer layer comprising metal, ceramic paper, cardboard orplastic material.
 18. The device according to claim 17, wherein themetal comprises zinc.
 19. The device according to claim 13, furthercomprising a vibrator arranged in, on or next to the cover.
 20. Thedevice according to claim 19, wherein the vibrator comprises a materialwhich forms gas between 100° C. and 1700° C.
 21. The device according toclaim 19, wherein an intermediate space is arranged between the vibratorand the cover, the intermediate space being smaller than an oscillationamplitude of the vibrator.
 22. The device according to claim 19, whereinthe vibrator comprises irregularities arranged in succession in alongitudinal direction on an outside of the cover, and an obstaclearranged next to the cover, such that the obstacle engages theirregularities.
 23. The device according to claim 22, wherein theobstacle is arranged on a fiber guide arrangement.
 24. The deviceaccording to claim 13, wherein the optical fiber is surrounded by ametal sleeve as an inner layer.
 25. The device according to claim 13,wherein the layers of the cover are arranged directly against oneanother.
 26. The device according to claim 25, wherein an innermost oneof the layers rests directly against the optical fiber.